1. Field of the Invention
The present invention relates to a data transmission method, a data transmission apparatus, and a data transmission system.
2. Related Background Art
Recommendation Series H.32x standardized by ITU-T (International Telecommunication Union-Telecommunication Standardization Sector) has been known as a real-time video communication method for visual telephones. According to this method, plural items of input data such as video data and audio data are combined into one item of data which is then communicated. Various expansion models are defined in Recommendation Series H.32x. In one of the recommendations, i.e., Recommendation H.324 Annex H, a definition is made on multilink communication in which one item of data is transmitted using a plurality of links (channels). For example, techniques associated with the recommendation H.324 Annex H are disclosed in an article titled “ITU-T Recommendation on CD-ROM”, Disc-2, T-REC-H.324-200011-I!AnnH!MSW-E.zip, November, 2000 (zip file) and March, 2000 (CD-ROM).
A functional constituent for processing video and audio data by adopting Recommendation H.324 Annex H will now be described with reference to FIG. 11. A video encoding section J1 encodes an input image using a video encoding method such as MPEG4. An audio encoding section J2 encodes an input sound using an audio encoding method such as AMR. A communication control section J3 performs control over the communication system such as switching control in terms of capabilities of the communication apparatus by utilizing messages defined in Recommendation H.245. A multiplexing section J4 adopts a multiplex process according to Recommendation H.223. A mobile multilink section J5 for converting the video and audio data into one item of multiplex data performs a multilink process according to Recommendation Annex H on the multiplex data obtained through the conversion to divide it into plural items of multiplex data.
A procedure for the multilink process at the mobile multilink section J5 will now be described with reference to FIG. 2. First, multiplex data D1 is divided in SS (sampling size) bytes, and the resultant items of data are sequentially distributed to buffers for respective channels on which they are to be transmitted. When items of data each having SS bytes are accumulated in a quantity corresponding to SPF (samples per frame), those items of data are given a synchronization flag F, header information H, and CRC information for the header information to generate a multilink frame MF, and the multilink frame MF is transmitted to another terminal. As shown in FIGS. 3A and 3B, the header information H includes full header information H1 and compressed header information H2. Those pieces of header information H include a number (CT) indicating the channel on which the multilink frame is to be transmitted, a flag (L) indicating that the multilink frame is transmitted on the channel having the greatest CT value among the channels to be used as transmission channels for multilink communication, a number (SN) indicating the place of the multilink frame in the order of transmission, and a flag (FT) indicating the type of the multilink frame. The full header information H1 further includes the SS and the size of the SPF.
A procedure for a process of restoring the divided data performed at the mobile multilink section J5 will be described. First, a synchronization flag is searched for each channel and, when a synchronization flag is detected, the SN and L in the header information following the flag are referred to. At this time, if those channels have the same SN, it can be decided that multilink frames are items of data which have been transmitted at the same timing. When there is a flag L in an “ON” state, it indicates that the relevant channel is the last channel. Thus, items of data transmitted over the respective channels are synchronized, and multiplex data which have been transmitted over the channels on a divided basis can be restored to obtain the initial multiplex data. Since the number SN can have values from 0 to 7 only, synchronization may not be achieved depending on the magnitude of a transmission delay. Therefore, the SS and SPF values a reset taking into account the communication environment and the size of a buffer at the receiving end. The restored multiplex data is divided into video data and audio data according to a method in compliance with the recommendation H.223 and into items of data according to the recommendation H.245, each of the resultant data being decoded. The items of decoded data are simultaneously played to allow real-time video communication.
During the above-described multilink communication according to Recommendation H.324 Annex H, a plurality of items of data (video data, audio data, and so on) to be transmitted on each channel on a divided basis are transmitted by storing them in a multilink frame. The volume of data stored in the multilink frame is determined by the SS and SPF values included in the header information as described above. It is desirable that the SS and SPF values are varied in accordance with changes in various factors such as the state of transmission of the channel in order to improve transmission efficiency. For example, when burst errors are likely to occur, a small SS value may be set to reduce the influence of burst errors. In the case of the occurrence of a delay in a channel, the value of SS′ SPF (payload) maybe increased to increase the amount of allowable delay.